Q 12-09-197JEE MainJEE Main 2019 (10 Apr, Shift 2)Medium
The graph shows how the magnification $m$ produced by a thin lens varies with image distance $v$. The focal length of the lens used is
Answer: (A) $\dfrac bc$
For a lens, $m = \dfrac vu$ and $\dfrac1v - \dfrac1u = \dfrac1f$. Multiplying the lens equation by $v$: $1 - m = \dfrac vf$, so the magnitude of magnification varies linearly with $v$ with slope $\dfrac1f$.
From the graph the slope is $\dfrac cb$, so
$$f = \frac bc$$
Solution by Sreeraj P, M.Sc Physics